Liberal Democrats have sought to douse the flames after a senior aide to Nick Clegg departed with a warning to David Cameron that his plans to reduce the size of the House of Commons would be thwarted if the prime minister fails to persuade Tory MPs to support Lords reform.

As Tory whips struggle to avoid a backbench rebellion next week, which could stop Lords reform in its tracks, Richard Reeves, who this week stood down as Clegg's director of strategy, warned of huge consequences if the bill falls.

But Reeves was accused by a senior Lib Dem of "completely overstating the case" after telling the Independent the Lib Dems would retaliate by blocking other constitutional reforms. The most significant of these are plans to shrink the House of Commons from 650 to 600 MPs, a reform that could give the Tories a net increase of up to 24 seats.

Reeves said prospects of the bill entering law would be "vanishingly small" if rebels rejected the programme motion. He said: "It is a very serious moment for the government. The vote is hugely significant. It is the critical moment for Lords reform, a once-in-a-generation chance to secure it.

"There would be broader consequences for the government's programme, particularly around political and parliamentary reform. The idea that failure to deliver a government commitment on Lords reform would be consequence-free is for the birds."

Reeves, a former Observer journalist who is moving to the US with his American wife, said Clegg would have no compunction about retaliating because the reform of the House of Lords lies at the heart of the coalition agreement. The reform bill, introduced by the deputy prime minister last week, "goes to the heart of what coalition is about – making and keeping deals".

Reeves added: "Anyone who thinks Nick Clegg will shrug his shoulders, say 'Never mind' and 'Everyone tried our best', will be in for a rude awakening. That is not going to happen. A deal is a deal."

The Tories are unlikely to welcome Reeves's intervention. They argue that the key constitutional trade-off in the coalition agreement was Cameron's offer to Clegg of a referendum on electoral reform in 2011 in exchange for the go-ahead for the Tories' plans to shrink the Commons.

Simon Hughes, the Liberal Democrats' deputy leader, later insisted that no decision had been made about a "quid pro quo" on reforms. "No decision has been made about that, no form of consultation," Hughes told BBC Radio 4's World at One programme. "Richard Reeves is completely overstating the case."

Hughes said there was not and never had been a link between Lords reform and plans to reduce the number of constituency seats, but he said parliament, each party and each MP "reserves the right to vote for or against the boundary commission recommendations when they come through".

It was "entirely reasonable" to look at reducing the number of MPs, but what had not yet been agreed were the awaited revised proposals put forward by the boundary commission.

"I know that colleagues in the Tory, Labour, certainly in the nationalist parties and in our party, are clearly nervous about boundary commission proposals, and there is precedent in the past when final proposals from the boundary commission come forward and parliament doesn't approve them. But that's an entirely separate matter, it's not linked to this issue at all."

In an appeal to Tory MPs, Hughes said he hoped "anybody who is in favour of a modern parliament and a modern democracy will vote in favour in this parliament of moving, carefully, slowly, gradually but moving to a place where parliament is principally elected in a second chamber by the people rather than being there by patronage or by heredity".

Up to 100 Conservative MPs, including a handful of ministerial aides, are planning to defy a three-line whip next week to vote down a "programme motion", which would establish a timetable for the Lords reform bill. Without such a motion the government's entire legislative programme could be disrupted for two years.

The Liberal Democrat MP David Laws played down suggestions that Lords reform would be derailed by the rebels. "I don't think that is remotely likely," Laws told BBC News. "All of the intelligence that we're getting is that the Conservative party remains committed to what was in their manifesto, the prime minister has given very clear leadership on this, it's a Conservative minister who's been working with Nick Clegg in devising these very sensible and measured plans, and so far, you know, the coalition – to everybody's surprise – made all these agreements on a whole range of policy areas back in May 2010."

Conservative rebels will reject the Lib Dem warning on the grounds that the coalition agreement simply said the government would establish a committee to "bring forward proposals" for a wholly or mainly elected upper house. They say they are therefore under no obligation to vote in favour of the reforms.

Downing Street reiterated that Cameron was committed to reforming the upper chamber.

"The prime minister has made his view on Lords reform very, very clear," his official spokesman said. "All of these issues, boundary changes, Lords reform, they are all set out in the coalition agreement and they are government policies that we intend to press ahead with. They are all government commitments."

The spokesman failed to rule out the option of a delay to the summer recess if MPs rebel next week. "Let's see what happens in the vote," he said.